1. Field of the Invention
This invention relates to a method for recovering cobalt, ruthenium, and aluminum from spent Co—Ru/Al2O3 catalyst.
2. Description of the Related Art
Renewable alternative liquid fuels are a possible solution to the problems of approaching oil depletion and increasing environmental pollution.
Fischer-Tropsch synthesis converts synthesis gas prepared by gasification of carbon-containing substances such as natural gas, coal, and biomass into liquid fuel (also called synthetic oil) with the aid of Fischer-Tropsch catalysts. Synthetic oil is a clean, renewable fuel that can be converted into gasoline or diesel by consequent treatment such as distillation.
The main metal active components of Fischer-Tropsch catalysts include the metal elements of Group VIII, such as iron, cobalt, nickel, and ruthenium. Cobalt-based catalyst is a hot topic in research and application of Fischer-Tropsch catalysts due to its outstanding catalytic ability. Ruthenium exhibits the highest catalytic activity, but its application is limited by its scarcity and high price. Thus, ruthenium is usually used as a promoter to improve the selectivity and activity of a catalyst.
Aluminum oxide has a high melting point, excellent thermal stability, and good wear resistance. It is widely used as a carrier for Fischer-Tropsch catalysts. For example, the weight percentage amount of aluminum oxide in the Fischer-Tropsch catalysts used in slurry bed is more than 50%.
Recovering cobalt, ruthenium, and aluminum from deactivated Fischer-Tropsch catalysts and recycling them into metal salts or oxides that can be used in preparation of catalysts is a method to reduce environmental pollution as well as production cost of the catalysts.
In one conventional method for recovering cobalt from cobalt-based catalysts carried on aluminum oxide, carbon monoxide is introduced into a reactor containing deionized water and spent cobalt-based catalyst containing SiO2, Al2O3, ZrO2, or TiO2 as carriers and heated at a constant temperature. Next, the reactor is cooled down and the carbon monoxide in the reactor is released. After that, the solution containing cobalt is released from the reactor and a lye is added to the solution to precipitate the cobalt therein as Co(OH)2. Nitric acid is added to the precipitate to dissolve it. After evaporation, Co(NO3)2.6 H2O is obtained. The Co(NO3)2.6 H2O obtained by this method has a purity lower than 99% and cannot he directly used in preparation of cobalt-based catalysts.
In another method for recovering cobalt, a spent cobalt-based catalyst on aluminum oxide carrier is ground, dissolved in concentrated hydrochloric acid, precipitated by sodium sulfide, precipitated by oxalic acid, calcined, dissolved in nitric acid, and crystallized by evaporation so as to obtain Co(NO3)2.6 H2O having a purity higher than 99%. However, because the metal oxide in the spent catalyst is not reduced and because the intermediate CoS produced in the recovery process is in a form of tiny particles, cobalt is likely to be lost during filtration, which leads to a low cobalt recovery rate around 92%.
Among the known methods for recovering ruthenium from spent catalysts, the most widely used one is an alkali fusion-oxidization distillation method for recovery of a ruthenium-based catalyst on an activated carbon carrier. In this method, the ruthenium-based catalyst is calcined at 600-1000° C. to remove the activated carbon carrier, and is then mixed with KOH and KNO3 and heated at 300-950° C. for 1-5 hours to conduct an alkali fusion reaction. After cooling, an alkali fusion product is obtained. The alkali fusion product is dissolved in water at a temperature of 50-90° C. to obtain a K2RuO4 solution. Then, sodium hypochlorite and concentrated sulfuric acid are added to the solution and refluxed at 50-90° C. for 2-4 hours to produce RuO4 gas. The RuO4 gas is absorbed by a strong acid solution and then distilled under atmospheric pressure or a reduced pressure to obtain a ruthenium salt. Because RuO4 produced in the distillation process is a strong oxidizer and is explosive and highly toxic, the reactions must be performed in a closed fume hood. Furthermore, the procedure of this method is complex and long.
There is another method for recovering ruthenium from a used catalyst containing ruthenium oxide and a carrier which is difficult to dissolve in inorganic acids. In this method, first, the catalyst is treated with hydrogen flow so that the ruthenium oxide in the catalyst is reduced to ruthenium metal. Next, the catalyst is treated with hydrochloric acid in an oxygen atmosphere so that the ruthenium metal on the carrier is dissolved. The obtained ruthenium (III) chloride solution is subjected to further treatment. This method has a low ruthenium recovery rate and is not suitable for recovery of catalysts containing γ-Al2O3 as a carrier.
The aforementioned methods are focused on recovery of a single metal rather than on recovery of cobalt, ruthenium, and aluminum simultaneously. Due to the different properties of the metals, the recovery rates and purities of the metals changes dramatically in accordance with the recovery method.